Writing bogus prescriptions is a felony. It is also fraud. I don't think you should just mind your own business. I think you better make it a mission to mind your own business. The consequences that happen to anyone with the drugs he is obtaining illegally are only secondary charges.
Here's the real point. Someones name is on the top of those prescriptions. They deserve to know. Some pharmacy is filling those prescriptions. The serious ripple effect to this can be life threatening. I would be no part of a felony or live with the burdon of knowing now if anything ever happens to anyone using those drugs.